Metakaolin Production and Enhancement of Industrial Minerals

ABSTRACT

Methods for preparing metakaolin-enhanced industrial minerals. Mixing and heating kaolinite clay with an industrial mineral, such as activated carbon, that is between 750° F. and 1400° F. results in a metakaolin/activated carbon complex that provides good mercury sorbent qualities while producing a fly ash (after use as a sorbent in emissions applications) that has a lower foaming index.

BACKGROUND OF THE INVENTION

1. Field of Invention

The subject matter herein relates to improvements in the production ofactivated carbon, metakaolin, cement clinker, and improved combinationproducts resulting therefrom, such as fly ash.

2. Description of the Related Art

Recent regulations governing the capture and reduction of mercuryemissions from coal fired applications have led to testing of numerousmaterials suitable for mercury capture. A leading mercury capturetechnology utilizes activated carbon which can be doped with, forexample, bromine

This activated carbon, when injected in the flue gas stream of acoal-fired furnace, will capture the mercury and in most cases befiltered out of the gas stream with the coal fly ash. This fly ash inmany applications, and most notably large utility coal-fired boilers, isoften utilized in the production of Portland cement.

Depending on the activated carbon source and characteristics, the addedcarbon content of the fly ash can adversely increase the foaming indexof the concrete, making the previously suitable fly ash fly ashunsuitable for concrete applications. Numerous investigations areunderway to produce an activated carbon with a low foaming index or todevelop an additive that can compensate for the foaming characteristicsof a particular activated carbon.

SUMMARY OF INVENTION

It has been discovered that when metakaolin and particularly highlyreactive metakaolin (HRM) is added to activated carbon, the activatedcarbon retained its mercury capture performance and the foaming index ofthe activated carbon was reduced.

Furthermore, the production of metakaolin and/or HRM by heating ofkaolinite clay (aka kaolin) as an initial co-product with one or moreother industrial minerals during a heat treatment such as calcination,or as an additive later in a heat treatment process (such that it isadded to previously heated and still hot industrial mineral material),is disclosed.

Additional features and advantages of the invention will be forthcomingfrom the following detailed description of certain preferredembodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Kaolinite is a clay mineral, a so-called industrial mineral, with thechemical composition Al₂Si₂O₅(OH)₄. When kaolinite clay is heated, itbecomes calcined by losing water through dehydroxilization. Thisdehydroxilization of kaolin leads to the formation of metakaolin with ageneral composition of Al₂O₃.2SiO₂+2H₂O with varying amounts of minorconstituents such as Fe₂O₃.

As used herein, High Reactivity Metakaolin (HRM) is defined asmetakaolin that contains SiO₂+Al₂O₃+Fe₂O₃ of >90%, that has a specificgravity of approximately 2.5, and that has a particle size less than flyash (FA) and greater than silica fume (SF). HRM is not distinct frommetakaolin in a chemical sense; rather, it refers to metakaolin that hasa higher purity and has been roasted from kaolinite to a tightercontrolled degree making it highly reactive.

The combination of the reduced foaming index and increased cementstructural characteristic through the addition of metakaolin andespecially HRM with activated carbon creates a unique product that isboth beneficial for mercury capture and concrete production.

A method to produce a metakaolin enhanced activated carbon includes theaddition (such as by mixing) of metakaolin in amounts ranging from 1 to30% by weight to brominated or non-brominated activated carbon. This canbe done prior to or after the activated carbon has been milled.

Another novel method to produce a metakaolin enhanced activated carboninvolves the addition of finely divided kaolinite clay to hot activatedcarbon after such carbon is discharged from a furnace or calciner andprior to final cooling. The production of metakaolin requires carefultemperature control to create the highest quality and reactivityproduct. The optimal temperature range is between 750° F. to 1400° F.and more ideally between 1000° F. and 1300° F.

Activated carbon discharging from a production method and cooled to nearthe temperature range above and mixed with finely divided kaolin claywill carefully calcine the clay using the heat provided by the hotactivated carbon. The coproduction of activated carbon and kaolin usingheat provided by the activated carbon upon discharge from a furnaceshould yield a high quality calcined metakaolin enhanced activatedcarbon. The amount of kaolin that can be added will be determined byseveral factors including activated carbon characteristics and kaolincomposition. In addition the amount of heat associated with activatedcarbon discharging from a furnace is a limiting factor. Supplementalexternal heat can also be added if required.

By including kaolin as a co-product industrial mineral to be calcined,metakaolin can be produced simultaneously with material such as cementclinker that can be a source of temperature control due to theendothermic nature of calcination (to better ensure that the kaolin isthoroughly heated to form metakaolin but not thermally treated in excessof optimal parameters).

So, for example, kaolin is introduced with the carbon being heat treatedin sufficient quantity that it results in at least 50% of the output (inthe form of metakaolin) in addition to the metakaolin doped activatedchar. Thus, a co-product metakaolin is to be distinguished from meredopants or “enhancers,” which are added to the system but do not resultin a separate “co-product” in any appreciable amounts (and certainly notover 50% of the system output of products). For example, adding abromide dopant to the carbon would be considered an enhancer and not aco-product industrial mineral because mainly brominated carbon results,with no other “co-product” making up at least 50% of the system outputof products.

In another example, hot cement clinker (between 1000° F.-1300° F.), thehot cement clinker being produced by sintering limestone andalumino-silicate in a cement kiln, is mixed with 5% kaolin by weightsuch that a metakaolin co-product results, for example after at least 30seconds with the hot cement clinker within the desire temperature range.The metakaolin enhanced cement clinker is further cooled and milled andprocessed to produce cement. While 5% kaolin is referenced in the aboveexample, between 5-20% could be added. Desirably, the enhanced cementclinker contains metakaolin of at least about 90% SiO₂+Al₂O₃+Fe₂O₃. Theuse of other industrial minerals, such as activated carbon, could beused in lieu or addition to cement clinker.

The utilization of flash activation to simultaneously calcine lime anddevolatilize and activate AC has proven very effective. Such a productis suitable for SO₂ and Hg removal in power plants.

Thus, a further example of novelty is a method of sorbing a contaminantin a flue gas, comprising introducing a metakaolin-doped activatedcarbon into a flue gas such that fly ash containing saidmetakaolin-enhanced activated carbon is produced. This can beaccomplished by Activated Carbon Injection (ACI) of the metakaolin-dopedactivated carbon into the flue gas duct prior to ash particulatecollection of a solid fuel fired industrial application, such as a powerplant.

HRM can be made by the invention by utilizing purified kaolinite ofgreater than 90% Al₂Si₂O₅(OH)₄ and thermally treating such materialunder controlled conditions where the thermal treatment temperature doesnot vary by more than 50° F. and preferably less than 20° F. from theoptimal desired temperature. The optimal treatment temperature isdetermined based on raw kaolinite characteristics and can vary fromdifferent kaolinite sources. The optimal temperature range is between750° F. to 1400° F. and more ideally between 1000° F. and 1300° F. Withkaolinite purities less than 90% Al₂Si₂O₅(OH)₄, but greater than 70%, orthermal treatment temperatures varying greater than 50° F., but stillbetween 750° F. to 1400° F., it is still possible to make a metakaolinenhanced industrial mineral product.

For embodiments involving the production of metakaolin enhancedindustrial mineral, it should be understood that the metakaolin or HRMand the industrial mineral, such as activated carbon or cement clinker,remain intimately mixed, and are not separated, producing a metakaolinand/or HRM enhanced industrial mineral product.

Various modifications are possible within the meaning and range ofequivalence of the appended claims.

1. (canceled)
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 5. A method ofproducing metakaolin, comprising the step of heating kaolinite clay asan initial co-product with one or more other industrial minerals duringa heat treatment process at a temperature and for a time sufficient tocreate metakaolin.
 6. The method of claim 5, wherein the one or moreother industrial minerals comprise activated carbon.
 7. The method ofclaim 5, wherein the one or more other industrial minerals comprisecement clinker.
 8. The method of claim 5, wherein said kaolinite clay isintroduced with the other one or more industrial minerals being heattreated in sufficient quantity such that at least 50% of the output ofthe heating process is in the form of metakaolin.
 9. The method of claim6, wherein said kaolinite is introduced in sufficient quantity with saidactivated carbon such that at least 50% of the output of the heatingprocess is in the form of metakaolin in addition to producingmetakaolin-doped activated char.
 10. A method of producing metakaolin,comprising the step of heating kaolinite clay with a hot industrialmineral, wherein said kaolinite clay is added to the hot industrialmineral after said industrial mineral has been heated to a temperatureof between 750° F. to 1400° F. and discharged from a furnace orcalciner, and the kaolinite clay is heated soley using the heat from thehot industrial mineral for a time sufficient to create metakaolin. 11.The method of claim 10, wherein said kaolinite clay is about at least90% pure and the temperature of the kaolinite clay does not vary by morethan 50° F. during the heating for a time sufficient to createmetakaolin.
 12. The method of claim 10, wherein said hot industrialmineral is cement clinker.
 13. The method of claim 10, whereinmetakaolin is produced in sufficient quantity to be a co-product. 14.The method of claim 12, wherein the hot cement clinker has been heatedto between 1000° F.-1300° F., and 5% kaolinite by weight is added tosaid hot cement clinker for about 30 seconds to thereby producemetakaolin.
 15. The method of claim 10, wherein said hot industrialmineral is carbon char.
 16. The method of claim 15, wherein the hotcarbon char has been heated to between 1000-1300° F., and 5% kaoliniteby weight is added to said hot carbon char for about 30 seconds tothereby produce metakaolin
 17. An enhanced cement clinker made accordingto the method of claim
 12. 18. The enhanced cement clinker of claim 17,wherein said enhanced cement clinker contains metakaolin of at leastabout 90% SiO₂+Al₂O₃+Fe₂O₃.
 19. A method of sorbing a contaminant in aflue gas, comprising introducing a metakaolin-doped activated carboninto a flue gas such that fly ash containing said metakaolin-enhancedactivated carbon is produced.
 20. A cement product made with theactivated-carbon containing fly ash produced by the method of claim 19.21. The method of claim 6, additionally comprising the step of addingkaolinite clay in an amount ranging from 1% to 30% by weight tobrominated or non- brominated activated carbon.
 22. A compositioncomprising activated carbon doped with metakaolin made according to themethod of claim
 21. 23. The method of claim 21, further comprising thestep of adding about at least 90% pure kaolinite clay to activatedcarbon that is at a temperature between 750° F. to 1400° F. and therebyheating said kaolinite clay for at least 30 seconds.
 24. The method ofclaim 23, wherein the temperature of the kaolinite clay does not vary bymore than 50° F. during the at least 30 seconds.